1. Field of the Invention
The present invention relates to an inkjet printing apparatus and an inkjet printing method which perform a color printing by applying ink or a reacting liquid, which makes a coloring material in the ink insoluble or coagulated, to a printing sheet, which has an air permeability from an obverse side of the printing sheet to a reverse side thereof (excluding a medium having for example a resin coated layer and then having no air permeability), such as a plain paper, a printing sheet having an ink receiving layer, and a converted paper.
2. Description of the Related Art
Recently, in a field of an ink printing art, an ink jet printing method has been studied and put into practical use in place of conventional printing arts. The ink-jet printing method has various advantages such as a low noise, low running cost, and facts in which a size of the apparatus is easily reduced, and colorization is easy, and is widely used in a printer or copier.
When, by these inkjet printing apparatus, an image is printed on a printing sheet which is a so-called plain paper, a fine streak (hereinafter, called feathering) along a fiber of the sheet is generated in a process in which the ejected ink penetrates in the sheet, and hereby, there may be a case where the sharpness of a monochrome text and a color image printed is impaired. Further, in the case that a color image is printed, the bleeding by a fact in which the ink is mixed in the boundary portion between different colors, is caused, and becomes a cause by which the quality of the color image is lowered. These phenomena are conspicuously caused in the case where the high speed printing is conducted, because the permeability of the ink constitutes a cause of the phenomenon, and therefore, the compatibility of the high speed printing and the high image quality is made difficult.
In contrast to this, Japanese Patent Application Laid-open No. 61-249755(1986) discloses an art in which, in addition to printing ink, a liquid (hereinafter, also called reacting liquid) including a hardening agent for insolubilizing or coagulating the ink is previously ejected as a liquid droplet to a location on a printing sheet, on which location the printing ink is to be landed, and the printing ink is reacted on the printing sheet. The gazette also discloses an advantageous effect that, by the printing ink being reacted, the feathering or bleeding by the ejected ink on the sheet is prevented, and the lowering of the quality of the image or print is prevented.
Further, Japanese Patent Application Laid-open No. 6-128514(1994) discloses a means that does not use the reacting liquid but makes respective pH of the black and color ink different, and cause the viscosity of the ink to be increased in the boundary between the black area and the color area in the image to prevent the bleeding.
Furthermore, in the Japanese Patent Application Laid-open No. 6-106841(1994), Japanese Patent Application Laid-open No. 11-334101(1999), Japanese Patent Application Laid-open No. 11-343441(1999), U.S. Pat. No. 5,428,383, U.S. Pat. No. 5,488,402, and U.S. Pat. No. 5,976,230, an ink-set which is formed of the black ink and color ink, and an ink-set in which at least one of color ink shows the nature which reacts to the black ink, and the other ink does not react to the black ink, and an inkjet printing system by using this ink set, are described. Then, the structure of this ink-set can make the bleeding decreased. Particularly, in order to prevent the bleeding in a boundary between a printing area by the black ink and a printing area by the color ink which shows the no-reactivity to the black ink, the printing method (hereinafter, called “under printing”) by which the printing of the color ink having the reactivity to the black ink is conducted on a printing area by the black ink in overlapping-manner, is disclosed.
As the black ink and the color ink which reacts the black ink used in the under printing, a combination of the black ink including a black coloring material having the hydrophilic radical and the reacting color ink including the reactive agent formed of metal ions, is known. When these inks are applied onto the printing sheet and mixed, and the hydrophilic radical is reacted to the metal ion, the black coloring material is caused to be insolubilized or coagulated. Hereby, it is prevented that the black coloring material moves to the printing area by the non-reactive color ink, which area adjoins the printing area by the black ink, and thus the bleeding generated between the printing area of the black ink and the printing area of the non-reactive color ink is decreased. Hereinafter, these ink-sets are called the reaction ink or reaction ink-set.
Any one of Japanese Patent Application Laid-open No. 61-249755(1986), Japanese Patent Application Laid-open No. 6-128514(1994), Japanese Patent Application Laid-open No. 6-106841(1994), Japanese Patent Application Laid-open No. 11-334101(1999), Japanese Patent Application Laid-open No. 11-343441(1999), U.S. Pat. No. 5,428,383, U.S. Pat. No. 5,488,402, and U.S. Pat. No. 5,976,230 shows a normal advantageous effect in the conventional art, which is brought by contacting liquid as droplets to each other at a boundary face to cause a desired reaction between the liquid droplets.
On the other hand, Japanese Patent Application Laid-open No. 2002-517341 discloses that the printing sheet for printing is applied with the reacting liquid over a whole surface by using a roller, which liquid is for example the liquid disclosed in Japanese Patent Application Laid-open No. 61-249755(1986), and then ink is ejected to the surface to perform printing.
It should be noted that Japanese Patent Application Laid-open No. 2000-062259 discloses sucking an air to attach a printing sheet for preventing the printing sheet from floating. Further, Japanese Patent Application Laid-open No. 10-309803(1998) discloses executing sucking on a reverse side of the printing sheet to accelerate permeation of the printing ink, in stead of using ink of high permeability.
However, a system disclosed in each of Japanese Patent Application Laid-open No. 61-249755(1986), Japanese Patent Application Laid-open No. 6-128514(1994), Japanese Patent Application Laid-open No. 6-106841(1994), Japanese Patent Application Laid-open No. 11-334101(1999), Japanese Patent Application Laid-open No. 11-343441(1999), U.S. Pat. No. 5,428,383, U.S. Pat. No. 5,488,402, and U.S. Pat. No. 5,976,230 is that ejects ink to the printing sheet and causes the ink to be reacted to insolubilize a coloring material in the ink or to increase the viscosity of the ink. For this, when ink or a reactive group in the reacting liquid reacts, an insolubilized matter or a viscous matter stays on a surface of the printing sheet. Then, this insolubilized matter or the like causes unevenness in optical density of a printed image, and prevents in some degree that a solvent or water included in the ink permeates into the printing sheet. As the result, there is a problem that the drying time or fixing time of the ink ejected on the printing sheet is prolonged. Hereupon, this drying time or fixing time indicates a time of about 3 sec–20 sec, which is, after the printing, a time from the time in which the printing sheet is discharged, to the time in which the next printing sheet is ejected. Further, as to a judgment whether the ink on the sheet surface is fixed or not, for example, in the case where the printing sheet is further overlapped on the printed sheet, when the ink is not transferred onto the other sheet, it is judged that the ink is fixed. Further, this fixing time is different depending on the printing duty (the ink ejection amount per unit area) for the printing sheet.
Further, the printing method described in each of the above Japanese Patent Application Laid-open No. 61-249755(1986), Japanese Patent Application Laid-open No. 6-128514(1994), Japanese Patent Application Laid-open No. 6-106841(1994), Japanese Patent Application Laid-open No. 11-334101(1999), Japanese Patent Application Laid-open No. 11-343441(1999), U.S. Pat. No. 5,428,383, U.S. Pat. No. 5,488,402, and U.S. Pat. No. 5,976,230, in which the reacting liquid is used, generates the insolubilization or coagulation of the coloring material by the reaction of the ink with the reacting liquid. Since this insolubilized matter or the like hardly penetrates into the printing sheet, many of those insolubilized matter or the like is remained on the surface of the printing sheet. For this, there is a case where, when the printing surface is rubbed with the printing surface of the other sheet, many insolubilized matters or coagulated matters on the surface are physically taken off, thereby, the quality of a printed material is degraded. More specifically, when the reacting liquid is used, although the solidity of the image such as the rubbing resistance is more increased than a case of the printing in which the reacting liquid is not used and only the normal ink is used, as the result in which much of reacting liquid remain on the surface and the insolubilized matter is formed. Then, the rubbed and damaged amount is relatively increased. Further, since many of the insolubilized matter or the like stay on the surface of the printing sheet, there are many cases where a film formed with stayed insolubilized matter or the like becomes un-uniform, and as the result, there is, sometimes, also a case where the fluctuation of density is generated in the printed image. The inventors of this application have found out that the cause of the fluctuation of density is that much of the reacting liquid remains on the surface of the printing sheet, and is uneven distribution of the reacting liquid which is controlled by a condition of fibers in the printing sheet.
Furthermore, the inventors have found out that, even if the system for applying the reacting liquid by means of the roller applies the reacting liquid even on the printing sheet, the permeation of the reacting liquid may fluctuate at the surface of the printing sheet or at a layer near the surface, depending on the condition of the fibers in the printing sheet. Particularly, there may exist portions including no reacting liquid and portions including much more reacting liquid inside the printing sheet, irregularly. Further, as a result, the application of the coloring material is fluctuated according to the fluctuated distribution of the reacting liquid and thus unevenness in fixing the coloring material is caused. For this reason, the print density distribution becomes what is far removed from a desired distribution, and thus the print quality becomes degraded. On the other hand, in the case of applying great amount reacting liquid, the opposite effect that an image is formed only on the surface of the printing sheet may be caused and thus an image quality is noticeably degraded.